Thermally diffused copper and zinc plate on ferrous articles



Jan. 8, 1946. H. BROWN ETAL 2,392,456

THERMALLY DIFFUSED COPPER AND ZINC PLATE ON FERROUS ARTICLES Filed July 16, 1942 67% (21. /5o f/rs.

Mug l xkwbm 5km I00 Z Zn.

Patented Jan. 8, 1946 THERMALLY DIFFUSED COPPER AND ZINC PLATE ON FERROUS ARTICLES Henry Brown and Carl R. Hurley, Detroit, Mich., assignors to The Udyllte Corporation, Detroit, Mich., a corporation of Delaware Application July 16, 1942, Serial No. 451,156

3 Claims.

This invention relates to the formation of brass coatings on steel or ferrous surfaces by thermal diffusion of copper and zinc layers deposited in such a manner and ratio as to obtain (after diffusion) a brass coating superimposed on a remaining copper layer.

We have found that if copper is plated first on steel and then zinc over it and the two layers diffused, the corrosion protection afforded steel by the resulting coating varies greatly with the ratio of copper to zinc, and when the ratio was below a certain percentage of copper, no copper layer remained, at which point the corrosion resistance afforded by the plate starts to deteriorate rapidly. Thus, the invention contemplates forming brass by thermal diffusion of copper and zinc layers of such ratios that after diffusion is complete, a copper layer still exists under the brass layer. The ratio of the thickness of the copper layer to the zinc layer and the order of plating are critical conditions to obtaining such a composite coating, The main copper coating must be plated first and must consist of very closely to 67% or more by weight of copper of total plate. Preferably the copper should be from 68% to 83% by weight of the total plate though the results are very significant from 67% to 90%. Such plates show remarkable corrosion resistance compared to copper plates of equal and even greater thickness; whereas, brass plates formed, for example, of 60% copper by weight of total plate are actually worse than copper plate of equal thickness. That is, while a plate of .001" copper on steel will fail in about 80 hours in the salt spray, a thinner plate of properly diffused 70-30 ratio of copper to zinc (or 80-20 ratio or in between ratios) will not fail in 200 hours in the salt spray; and a similarly diffused plate on the same steel of 60-40 ratio will fail in about 24 hours.

In other words, we have found that as soon as a critical ratio of copper to zinc is exceeded, i. e., copper less than 67% by weight of total plate, the corrosion resistance afforded by the plate drops suddenly and enormously. How. ever, if the zinc is plated first and then the copper and the ratio is 70% copper to 30% zinc, the plate will also fail in 24 hours in the salt spray. Thus, not only is the ratio of copper to zinc important, but also the order of plating. Thus, while it is not new in the art to form brass coatings by thermal diffusion of copper and zinc layers (cf. M. Rubin, U. S. Patent No, 2,115,749, May 3, 1938), nevertheless, as far as I am aware, there is no evidence in the literature which indicates the absolute importance from the standpoint of corrosion resistance afforded by the plate of the order of plating (i. e., of copper first before the zinc), and the fact that the copper must constitute at least 67% of the total weight of the plate or else the corrosion resistance of the resulting plate is exceedingly poor. Of course, from the standpoint of indoor decorative purposes or where very thin plates are used, as with ordinary brass baths or as in Rubins example in strip plating (the plate is less than .00025") these distinctions are not nearly as important.

The relation of corrosion protection to composition can be best illustrated by the following example. If .0005" of copper is first plated on two similar steel specimens and then one followed With .00025" of zinc and the other allowed to plate longer in the'zinc bath to. for example, .0004" zinc, it is found that after diffusion, the thinner coating will give exceedingly better corrosion protection of the steel, for example, in the salt spray the thinner coating will stand at least 200 hours whereas the thicker one,only about 18 hours before failure, and furthermore, on longer standing, in the salt spray, the thinner may de- Velop only 2 or 3 spots per sq. ft., the thicker will usually have over 30 in 24 hours.

Thus, it has been found that when the ratios of diffused copper and zinc plate are such as to total more than 33% of zinc, the corrosion protection afforded steel, as illustrated for example, in salt spray results, becomes inferior to plate of equal thicknesses of copper, whereas the ratios of diffused plate containing less zinc make possible coatings that greatly exceed the corrosion protection afforded by copper of equal and even greater thicknesses. This is true, as has been mentioned, providing the copper is plated on the steel before the zinc is put on. If the zinc is plated on first, then the bond obtained and the corrosion resistance are very poor, e, g., .00075" plate will fail in about 24 hours or less in the salt spray, while if it had been plated with copper first and zinc second, and then diffused, the plate would not have failed in 300 hours. Where the zinc is plated on first and the copper plated on the zinc, very poor adhesion. and very poor corrosion resistance results whether an acid or cyanide zinc bath is used. If the acid bath is used first, the entire plate blisters off in large blisters during the diffusion, but with cyanide zinc no blisters are usually obtained but the bond of the diffused plate to the steel is poor and as mentioned, the corrosion resistance of such a plate is very poor, even if the ratio of copper to zinc is 70-30, for example. Also, when the zinc is plated first and the copper second, and the ratio is 70 copper to 30 zinc (by weight), then after diffusion, no copper layer exists against the steel but the brass which exists down to the steel has poor bond and when the plate is peeled off, the steel appears etched.

It is preferred that the baths used in the electroplating of the copper and zinc do not contain too much of any addition agents which may plate out, for example, such percentages of sulfur, selenium, carbon, organic matter, basic salt inclusions, lead or tin, etc., as to cause blistering in the diffusion process. Small amounts of addition agents which promote throwing power or grain refinement or cut down tarnish or aid in th corrosion resistance are not necessary harmful, but larger amounts may be in the diffusion process, especially is this true in the formation of thicker plates. The copper can be plated from well known copper baths such as copper cyanide types, acid copper sulfate (after copper cyanide flash), ammoniacal copper pyrophosphate, etc., and the zinc can also be plated from well known baths such as the zinc cyanide and acid zinc baths as well as otherknown and possible zinc baths. The thicker the plate desired to be diffused, the more essential is it to have plate that is free from inclusions and foreign matter (unless a hydrogen atmosphere is used in the diffusion which may help in certain cases) and for these conditions cathode efficiencies as close to 100% as possible are desirable and often necessary. In making thinner deposits, however, of the order of .0005", it is often desirable to have brighteners in the bath to get a brighter-looking finish after the diffusion. For example, if an acid copper bath, using traces of thiourea is used to form the bulk of copper plate, a very fine-grained, hard, rather brittle copper deposit is obtained, but when this copper deposit is plated over with zinc in proper ratio and then subsequently diffused (especially in the hydrogen atmosphere) the resulting coating is a very ductile, high corrosion resistant brass. Whereas, if cyanamide is used in the acid copper bath to obtain a very fine-grained, hard, rather brittle plate, the plate is not improved nearly as much by the same treatment just described. In fact, annealing these fine-grained copper plates alone in the hydrogen furnace also shows the difference in the action of the brighteners and the products left in the copper plate, because the ductility of the copper plate obtained with the thiourea (or similar type) addition agent is greatly improved while the plate obtained with cyanamide is not.

In the plating of recessed parts, it is often of considerable importance to consider the type of copper and zinc baths to use from the standpoint of their relative throwing powers, in order not to exceed a ratio of 33:67 in zinc to copper in recesses.

Of importance is the fact that the steel can be first plated with nickel (bright or dull) and then plated with copper and zinc in proper ratio which is then diffused to form a composite coating of I very high corrosion protection value. In this case, smaller thicknesses of the diffused copperzinc layer may be used with the nickel undercoat present. Also, if thin, fine-grained layers of cop per and zinc are diffused over an undercoat of with nickel or bright nickel and then chromium to give white decorative finishes of exceedingly high corrosion protection to the underlying ferrous metal, much better than a similar undercoat of copper can give. Also, for other types of corrosion protection lead can be plated over such properly diffused brass coatings. In these cases, even thin coats of lead of the order of .0005" over such a brass coated steel will give amazingly high salt spray resistance (500 hours and more), much better than lead of thicknesses equal to the total plate over the steel itself. Also, excellent results are obtained with lead-tin alloys and also with tin itself over such an undercoat of the diffused plate.

For thicknesses of .0003" and heavier, i. e., .0005" to .002", the best temperature for diffusion is from about 700 F. to 1000 F. though higher temperatures can be used except in the case of deposits which are sufficiently thick to allow zinc to melt before it can diffuse and hence, in such cases temperatures higher than the melting point of zinc are not desirable. The time for diffusion is from a few minutes at the higher temperatures and for thinner plates, to about an hour or over for thicker plates and lower temperatures. For a thickness of .0005 to .001", it is preferred to use from 700-950 F. and from 30-50 minutes for time of diffusion.

The use of a non-oxidizing atmosphere is not essential, though desirable especially with thicker plates. In case an air atmosphere is used, it is best to plate in the following manner, especially for the thicker plates. Plate correct amount of copper and zinc, then flash with copper (5-30 seconds) and then flash with zinc (5-30 seconds). In this way, the immediate formation of a brass film from the two top thin layers prevents the oxidation from going on as rapidly as it would otherwise during the time it takes for the diffubright nickel, the resulting brass coat will be bright also and necessitate little or no bufling.

Also, the copper-zinc coating can be electroplated 7 sion to come to equilibrium. In this way, the plate comes out of the oven with a green or yel 510w to brown color, depending on how thick the flashes of copper and Zinc were and the temperature used in the diffusion of the work. The

color can also be varied by using a single copper .flash or by varying the thickness of the copper and zinc flashes. An acidic chromate dip of correct composition and ratio of chromate and sulfate (or other suitable anion) and pH range will give excellent tarnish and corrosion resistance of the brass coat itself, for example, in salt atmospheres when applied to the oxidized surface obtained by diffusion in air. The color of such a chromate coat (containing much zinc chromate or basic chromates) is like brass itself, and it is important in the dipping process not to immerse the work too long to dissolve all of the oxide off because the type of coating then formed is not nearly as protective. In case the brass metal color is desired or it is desired to plate on the brass (as for example with nickel), it is necessary to acid dip or. use a bright dip or an electrolytic method to remove the oxide film. Of course, if a hydrogen or non-oxidizing atmosphere is used,

the brass color can be obtained right from the must plate copper to zinc in the ratio of 66.5 to 33.5 thicknesses. For example, for .001" plate (actually there is a volume change), we must use .00066" copper and .00033" zinc to obtain a 71:29 ratio (not the exact ratio of the brass itself as a thin copper layer will remain against the steel with such a ratio of copper to zinc). At about 67% copper (67% of total plate), this thin copper layer will Just disappear, and it is with copper contents lower than 67% that corrosion resistance will begin to drop rather suddenly as for example, at 65:35 or 60:40 the plate is no longer even equal to copper plate of equal thickness in corrosion protection.

The curve or graph shown in the drawing 11- lustrates the relation of salt spray resistance afforded steel samples coated with .001" plate of copper-zinc diffusion plates with their percentage composition. The preferred ratio is from 68% copper to about 80% copper with the balance zinc with 68-73% copper balance zinc as the optimum for highest corrosion protection afforded to steel or ferrous surfaces.

We claim:

1. The method of coating a ferrous surface comprising electrodepositing a copper coating on said surface, thereafter electrodepositing a zinc coating on said copper deposit, the ratio of the thickness of the copper coating to the thickness of the zinc coating being such that the copper coating comprises from 68% to 73% by weight of the combined weight of the'copper and zinc deposits, and then heating the two deposits at alloying temperatures to cause some of the copper and all of the zinc to alloy at a ratio such that of the total diffused copper and zinc less than 33% is zinc whereby a brass coating is formed over a copper coating.

2. The method of coating aferrous surface comprising electrodepositing a copper coating on said surface, thereafter electrodepositing a zinc coating on said copper deposit, the ratio of the thickness of the copper coating to the thickness of the zinc coating being such that the weight of the copper coating falls within a range of from 68% to 83% of the combined weight of the copper and zinc deposits, and then heating the two deposits at alloying temperatures to cause the copper and zinc to alloy at a ratio such that'of the total diffused copper and zinc less than 33% is zinc whereby a brass coating is formed over a copper coating.

3. The method of coating a ferrous surface comprising electrodepositing a copper coating on said surface, thereafter electrodepositing a zinc coating on said copper deposit, the ratio of the thickness of the copper coating to the thickness of the zinc coating being such that the weight of k the copper coating falls within a range of from 68% to 73% of the combined weight of the copper and zinc deposits, and then heating the two deposits at a temperature falling within a range of from 700 F. to 1000 F. for a period of time falling within a range of from about fifteen minutes to about fifty minutes whereby to effect an alloying of some of the copper and the zinc at a ratio such that of the total difiused copper and zinc less than 33% is zinc whereby to give a layer of brass over a remaining layer of copper.

HENRY BROWN. CARL R. HURLEY. 

